The size of a molecular weight of a polymer exerts a strong influence on the chemical properties and the physical properties of the polymer.
In general, as the molecular weight of a polymer grows larger, shown are tendencies such as an increase in the strength of the polymer, a reduction in blocking and an improvement in the weatherability. On the other hand, as the molecular weight of a polymer decreases, shown are tendencies such as an improvement in a heat-melting property, a heat fluidity, an adhesive property to various materials and a penetrability of the polymer and a gloss of the paint film formed.
Making use of these characteristics of polymers having a low molecular weight, the polymers having a low molecular weight are used in large quantities for molding resins having a good fluidity, electrophotographic toners, base materials for hot melt type coating agents, coating resin-modifying agents having good penetrability and wetting property against substrates, and tackifiers. Further, since the polymers having a low molecular weight have a good mixing property or dispersibility in various materials, they are useful for specific uses such as pigment dispersants, mineral dispersants, water-treating agents in boilers, cooling tower, reverse osmosis treatment, sugar refining, paper making, geothermal treatment, oil wells and the like, and detergent additives acting as builders, film-forming preventives, sequestering agents and adhesion-inhibiting agents.
Such polymers having a low molecular weight are obtained by polymerizing various monomers by methods such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization and the like, and they are produced preferably by emulsion polymerization because of the reasons that polymerization can easily be controlled and the resulting polymers are easy to handle.
Usually, when polymers having a low molecular weight are obtained by emulsion polymerization, the polymerization has so far been carried out in the coexistence of a chain transfer agent in order to reduce the molecular weight. In general, in this case, aliphatic mercaptan base and halogenated hydrocarbon base chain transfer agents are mainly used as chain transfer agents for industrial purposes.
However, a polymer dispersion obtained in the presence of mercaptans has undesired specific odor even if the remaining mercaptan content is small. When halogenated organic compounds (for example, carbon tetrachloride, bromoform, bromotrichloromethane and the like) are used as a chain transfer agent, the content of the chain transfer agent remaining in the polymer dispersion is relatively much remained, so that problems on air pollution and toxicity are brought about.
A principal object of the present invention is to produce a polymer of a low molecular weight having no or little odor by an emulsion polymerization method.
Intensive researches continued by the present inventors have resulted in finding that a polymer of a low molecular weight having no or little odor can readily be produced by emulsion-polymerizing a radical-polymerizable unsaturated monomer on a higher polymerization temperature condition than a polymerization temperature usually used for emulsion polymerization, and thus have come to complete the present invention.